A new approach to the problem of failure prediction of wood components and structures is that of fracture mechanics. In order to implement this approach knowledge of flaw growth (or crack propagation) in wood subjected to various stress systems and environmental conditions is required. The extension of flaws is accompanied by the release of energy in the form of acoustic or stress wave emissions, which can be detected and have been found to be reliable indicators of crack growth. Using acoustic emissions as a measure, flaw growth was investigated in stressed bending specimens of three species. The test pieces contained a variety of knot configurations, seasoning checks and resin pockets. Three wood moisture contents were used; specimen size was 2-by-4-inch (nominal) cross section and 50-inch length. A detailed description of the system used for detecting, measuring and recording acoustic emissions is given. Emission activity was measured using an electronic counter. Simultaneous count, load and deflection measurements were recorded and count-deflection and load-deflection curves plotted. The association between acoustic emissions and crack growth is discussed. Four types of integrated count- deflection patterns were found, which generally exhibited two common features. Firstly, a rate change point, where the count rate increased rapidly, and secondly a count increase just prior to failure. The rate change point was approximately coincident with the proportional limit in the load-deflection curve, suggesting an association between elastic behaviour and microfailure development. Several qualitative relationships between the count and load curves are presented. A correlation between stable crack growth and the entire load-deflection curve is postulated. The presence of checks and resin pockets affected the microfailure pattern, shown by increased acoustical activity, without appreciably influencing modulus of rupture. Green specimens for the most part produced lower counts than drier ones. Also clear or essentially clear boards exhibited lower emission counts. Species differences were not apparent. The estimation of modulus of rupture employing parameters from the count-deflection curves is discussed and compared to that using modulus of elasticity. Precision of the prediction was not improved using count parameters. A method for determining the direction of crack propagation in a fractured bending member is described. / Forestry, Faculty of / Graduate

The effect of specific active environment on the fracture strength of glass and polymethyl methacrylate was investigated using an indirect tensile testing technique. The strength of glass was not affected by exposure to dry gaseous N₂ and CO₂. At low water vapour coverages, (<1/3 monolayer), the tensile strength of glass was reduced by approximately 50%. Further increase in water vapour pressure did not weaken the solid to a much greater extent. The existence of surface microcracks governs the absolute tensile fracture strength, and any process which varies the flaw geometry acts to vary the tensile fracture strength. Soaking in the liquid has the same effect as adsorption from the vapour phase near saturation. All vapour adsorbates caused a weakening, the magnitude of the decrease increasing with increasing ability of the adsorbate to screen the surface Si++++ cores. Moisture was the most active environment encountered. Polymethyl methacrylate did not weaken in the vapour phase despite multilayer adsorption, but stressing in wetting liquids did cause drastic failure, with a 57% decrease in tensile strength. Non-wetting liquids do not affect the strength of the acrylic plastics. Fracture experiments on a quartzitic rock in aqueous solutions of surfactant, (quartenary ammonium salts), show that the weakening due to surfactant adsorption is negligible, since water itself causes the maximum strength reduction. The adsorption of surfactant is only a secondary effect. A mechanism has been proposed for the stress-environmental failure of brittle solids. This mechanism recognizes the existence of micro-cracks, regards the stable crack propagation stage of the fracture process to be environment sensitive, and involves the strain-activated adsorption resulting in a decrease in cohesion at the flaw apex. The magnitude of the weakening is critically dependent on the nature of the bonding in the solid surface. A literature review of stress-sorption cracking, with an emphasis on non-metallic systems, is included. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

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Vita. / Digitized by Kansas Correctional Industries

published_or_final_version / abstract / toc / Civil Engineering / Master / Master of Philosophy

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